The present invention provides RF components which have reduced occurrence of multipactor effect in high power RF applications, and in particular, space applications. The present invention further provides a method for reducing Mulitpactor effect in high power RF applications and other high energy environments including applying a surface treatment or coating having a low secondary electron emission yield (SEY), or secondary electron emission coefficient.
The multipactor effect is an electron cloud in a vacuum that grows in avalanche in resonance with a high frequency electromagnetic field. The electron avalanche is accelerated by the field and is fed back by secondary electron emission produced by electrons impacting on exposed surfaces. This electron discharge limits the achievable power in RF devices working in a vacuum. Multipactor is a serious problem in fields of great technological importance such as high power RF hardware in space, high-energy particle accelerators, and klystrons and other high-power RF vacuum tubes. The multipactor resonance conditions can often be avoided by proper design of parameters pertaining the electromagnetic field; however, there remain always critical parts where multipactor can only be avoided by low secondary emission surfaces.
In the secondary electron emission process one electron impacting on a surface can transfer part of its energy to one or more electrons of the surface that are thus emitted back into vacuum. A secondary emission yield of more than one electron per impacting electron is needed for multipactor effect to be possible. The secondary emission yield directly influences in the risk of multipactor discharge.
A coating used to reduce the multipactor effect should have low electrical resistivity to avoid deterioration of the RF performance of the device. The requirements of low secondary emission and low surface resistance are, for physicochemical reasons, in contradiction with surface stability in air, this last requirement being of most importance for space applications. The best generally known coating material found to date is titanium nitride (TiN) which dominates in other applications (different from space), such as in vacuum devices. Deterioration of TiN surface properties because of long exposure to air is recovered by special treatments (surface conditioning by vacuum heat treatments or ion/electron bombardment) once the surface is under vacuum for operation.
TiN surface conditioning treatments are impossible or impractical in space applications. As a consequence, the best generally known coating found for these applications is Alodine for aluminium alloys; it is a chromate conversion coating for corrosion protection and as such it is very stable in air. However, its electrical conductivity is not sufficient for the ever increasing RF-performance requirements. A further drawback is the use of chemicals with the dangerous CrVI ion in the preparation of chromate conversion coatings like Alodine.